Intro: Circuit Bending Audio Greeting Cards
Everyone has seen, received or given one of these audio greeting cards. They play songs, quotes from movies or TV shows and some of them allow you to record your own message. They’re fun the first time. Maybe the second time too. By the third time, you’re afraid to open the card. What we’re about to do is make these things way more fun for a much longer time.
American Greetings/Carlton Cards Sound or Music Greeting Cards
22 AWG Wire (solid core just to make your life easier)
Saliva and Finger
Various Resistors (between 50KOhms and 150KOhms)
Various Potentiometers (between 50KOhms and 150KOhms)
knowledge of breadboard
Step 1: Pick a Card ....
Buy an audio greeting card
If you can pull the card open a little bit, around where the white strip/switch is, without ripping it, look for resistors. If you see some, you’re set. If not, try a different one. We recommend American Greetings or Carlton Cards, because we’ve ripped them open ourselves and those work out well.
After the card has been paid for, try and pull the card open around where the switch is until you expose the entire inside of the card.
Step 2: Open and Test
Testing what happens
This is the only time we’d recommend touching a circuit with a wet finger. Trigger the audio by just lifting the metal tab and letting go. While the audio is playing, wet the tip of your finger and rub around the solder points on the board. The areas that will be most responsive are around resistors and capacitors. There’s a chance that the sound might get stuck in a faster or slower playback speed. It’s not a big deal since you’ll be playing around with it more anyways and you'll wind up putting the audio back in its original state.
Step 3: Removing the Blue Resistor
These cards are all pretty standard. The blue resistor you see is about 200KOhms. The plan here is to replace that blue 200KOhm resistor with a comparable variable resistor. It can be just about anything that will vary resistance around 200KOhm. In the video below, we're using a 100K trimpot. Start out by desoldering the blue resistor. Take note of the solder bridge between the two resistors because that will need to be replaced. A solder bridge is what it sounds like. It's solder connecting two points that wouldn't normally be connected. Most of the time a solder bridge is a bad thing, but here the solder bridge is used to cut costs by not putting a connection trace on the board or using wire to connect the two resistors.
To desolder, grab your solder pump and hold your hot soldering iron to one contact of the blue resistor. When the solder melts, activate your solder pump and voila...solder gone. Do it again on the other contact. You may have to desolder the resistor a couple times to get it free, but you'll get it.
Step 4: Testing Your Variable Resistor
Take two pieces of wire and strip the ends. Since there's no solder pads on the bottom of the circuit board, we need to make sure there's exposed wire above the board where the pads are.
Put the wire in through the former home of the blue resistor's legs and bend the wire underneath being sure to leave a bit of exposed wire above.
Solder each wire in place. Once that's done, add solder to the same place where the solder bridge was as mentioned in the last step, to recreate the solder bridge.
Now that we've got two pieces of exposed wire, we can connect them to anything. In order to make testing easy, we're going to use a breadboard.
Choose your variable resistor. In the case below, we're using a 100K trimmer. Place your variable resistor on the breadboard. For potentiometers the wires should be connected to the middle and one of the outer connectors. For trimmers, one wire should be connected to the contact on the side with only one contact and the other wire connected to either contact on the side with two contacts.
Once everything's connected, trigger the circuit like we did earlier to test it and you should hear the speed & pitch completely change while you're turning the potentiometer or trimmer.
Step 5: Putting It Back Together
At this point, you should have figured out what kind of variable resistor you want to use. Rather than desoldering the wire we used, we're just going to solder the variable resistor to the wire.
Solder together your new component for pitch control.
If you'd like to hook together a different switch for this circuit, it's easy enough. Just cut the metal tab so it's not touching the contact beneath it. Then solder some wire to both the bottom plate and the remaining part of the tab. Finally solder a push button to the wire and you've got yourself a sweet little sample player. If you use a momentary push button instead of a toggle switch, then you can repeatedly start and stop the song whenever you choose.